• 한국전산구조공학회논문집
• /
• 제28권5호
• /
• pp.477-483
• /
• 2015

본 논문에서는 Euler-Bernoulli Beam(EB-beam) 및 신속 Fourier 변환을 이용하여 수치분석적 빔 모델 및 Co-rotational plane beam EDISON program(CR-beam)을 이용한 빔 모델의 가진주파수 변화에 따른 외팔보의 자유단 진동 연구를 수행하였다. 위의 두 빔 모델에서의 끝단에서는 진동이 시간이 지남에 따라 감소하다가 정상상태에 이르는 것을 확인하였다. 끝단에서 가진주파수가 증가함에 따라 구조적 감쇠에 의해 변위이 감소하는 경향을 보인다. 감쇠를 고려한 EB-beam과 CR-beam가 정상상태로 진입하는 경향이 비슷하나, 가진주파수는 정상상태가 나타나는 시간과 독립적임을 제시한다.

• 한국수자원학회논문집
• /
• 제36권6호
• /
• pp.949-960
• /
• 2003

정상상태 가정을 완화시킨 습윤지수의 해상도 문제가 수치지형모형에서 다루어졌다. 반 동역학적 습은지수와 동력학적 습윤지수의 변화성을 수치고도모형의 격자 크기와 배수시간을 변화시키면서, 공간적 통계적으로 고찰하였다. 변화하는 배수시간과 격자크기에 따라 습윤지수의 구조화 양상이 관찰 되었다. 설마천 유역의 적용결과 비교적 짧은 배수시간에서는 습윤도의 천이성이 관찰되었고, 10,000 시간 이상의 배수 시간에서는 통계적 분석 결과가 정상상태의 특성으로 수렴함을 보여주고 있다. 반 동역학적 및 동역학적 습윤지수의 확률밀도 함수들은 계산결과의 안정성과 일관성에 관련이 있는 경계 격자 크기가 존재함을 보여주고 있다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2007년도 춘계학술대회B
• /
• pp.2769-2774
• /
• 2007

In vacuum ejector-diffuser systems where a finite volume secondary chamber is used, the secondary jet exhibits transient characteristics during start-up. A steady state is achieved after some time in which mass entrainment prevails indefinitely inside the ejector, though there is no flow from the secondary chamber. An attempt is made in this work to study the infinite entrainment of secondary jet into the primary jet from a finite secondary chamber, with the help of a computational fluid dynamics method. The present study is also intended to identify the operating range of vacuum ejector-diffuser systems where the steady flow assumption can be applied without uncertainty. The results obtained show that the only condition in which an infinite mass entrainment is possible is the generation of a re-circulation zone near the primary nozzle exit. The flow in the secondary chamber attains a state of dynamic equilibrium at this point. Steady flow assumption is valid only after this point.

• 전기화학회지
• /
• 제10권3호
• /
• pp.196-202
• /
• 2007

본 연구에서는 고분자 전해질 연료전지의 가습조건의 변화에 따른 정상상태 및 비정상상태 운전특성을 살펴보았다. 전압을 OCV에서 0.25 V까지 변화시키면서 PEFC 연료전지의 정상상태 성능을 정전압 모드에서 측정하여 전류-전압 곡선에 나타내어 고찰하였다. 또한, 일부 전압조건에서 연료전지의 비정상상태 성능변화를 측정하였다. 수소극 가습에 따른 성능을 평가하기 위하여 공기극은 건조공기를 공급한 상태에서 수소극에 공급되는 수소의 습도를 20%에서 100%로 변화시키면서 연료전지의 성능을 측정하였다. 일반적으로 고전압 영역에서는 높은 작동온도가 높은 성능을 나타내고 있으나, 저전압 영역에서는 낮은 작동온도가 높은 성능을 나타내었다. 임피던스 측정을 통하여 건조한 전해질막 조건에서 ohmic 손실이 커지며 외부가습과 자체가습량이 커지면 저주파수 영역에서 물질전달손실 효과가 나타나는 것을 확인하였다. 또한, 전압을 감소시킨 후 전류의 시간에 따른 변화를 고찰하여 연료전지 시스템의 동적특성을 고찰하였다. 전압을 감소시킨 경우, 작동온도의 상승에 따라서 정상상태에 도달하는 시간이 줄어들었으며, 저전압 영역을 제외하면 생성된 물에 의한 자체가습은 정상상태 도달시간을 지연시키는 효과를 가져왔다.

• International Journal of Fluid Machinery and Systems
• /
• 제8권3호
• /
• pp.155-168
• /
• 2015

No-load speed is an important performance factor for the safe operation of hydropower systems. In turbine design, the manufacturers must conduct several model tests to calculate the accurate value of no-load speed for the complete range of operating conditions, which are expensive and time-consuming. The present study presents steady and unsteady methods for calculating no-load speed of a Francis turbine. The steady simulations are implemented using a commercial flow solver and an iterative algorithm that relies on a smooth relation between turbine torque and speed factor. The unsteady method uses unsteady RANS simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step and friction torque. The main goal of this research is to evaluate and compare the two methods by calculating turbine dynamic parameters for three test cases consisting of high and medium head Francis turbines. Overall, the numerical results agreed well with experimental data. The unsteady method provided more accurate results in the opening angle range from 20 to 26 degrees. Nevertheless, the steady results showed more consistency than unsteady results for the three different test cases at different operating conditions.

• Journal of Power Electronics
• /
• 제18권3호
• /
• pp.766-777
• /
• 2018

In this paper, a sliding mode and proportional plus integral (SM-PI) control combined with self-sustained phase shift modulation (SSPSM) for LLC resonant converters is presented. The proposed control scheme improves the transient response while preserving good steady-state performance. An averaged large signal model of an LLC converter with the ZVS modulation technique is developed for the SM control design. The sliding surface is obtained based on the input-output linearization concept. A system identification method is adopted to obtain the transform function of the LLC resonant converter, which is used to design the PI control. In order to reduce the inherent chattering problem in the steady state, the combined SM-PI control strategy is derived with fuzzy control, where the SM control is responsive during the transient state while the PI control prevails in the steady state. The combination of SSPSM and the SM-PI control provides ZVS operation, robustness and a fast transient response against step load variations. Simulation and experimental results validate the theoretical analysis and the attractive features of the proposed scheme.

• 한국추진공학회지
• /
• 제4권1호
• /
• pp.93-101
• /
• 2000

엔진 운용 중에 유로(flow Path)에 놓여 있는 팬 블레이드(blade)들은 많은 외부하중에 노출되어 있어서 고주기 피로(high cycle fatigue)에 의한 피로 파괴의 위험성이 크다. 그 중 가장 중요하게 평가되는 것이 주기적인 가진력에 의한 공진 현상과 그에 따른 피로파괴 가능성이다. 본 논문에서는 유동장 (flow field)이 지주(struts)에 의해 영향을 받게 되고, 이러한 유동장의 분포가 지주 후방에서 주기함수 형태로 팬 블레이드를 가진 할 때, 팬 블레이드에서의 진동 응답 특성과 구조적인 안정성을 평가하였다. 팬 블레이드의 피로강도를 시험적으로 평가하고, 팬 블레이드 전방 지주에 의한 공기 가진력을 가정하여 유한 요소 해석을 통한 구조적 안정성을 평가하였다. 그리고 엔진 시험에서 측정된 서지 압력 하중을 팬 블레이드의 유한요소 모델에 적용하여 구조적 영향을 평가함으로써 팬 블레이드의 구조적인 안정성을 확인하였다.

• 한국소음진동공학회논문집
• /
• 제20권1호
• /
• pp.22-28
• /
• 2010

Approximate analysis for a building installed with a friction damper is performed to get insight of its dynamic behavior. Energy balance equation is used to have a closed analytical form solution of dynamic magnification factor(DMF). It is found out that DMF is dependent on friction force ratio and resonance frequency. Approximation of DMF and equivalent damping ratio of a friction damper is proposed with such assumption that the building with a friction damper shows harmonic steady-state response and narrow banded response behavior near resonance frequency. Linear transfer function from input external force to output building displacement is suggested from the simplified DMF equation. Root mean square of a building displacement is derived under earthquake-like random excitation. Finally, design procedure of a friction damper is proposed by finding friction force corresponding to target control ratio. Numerical analysis is carried out to verify the proposed design procedure.

• 한국자동차공학회논문집
• /
• 제17권4호
• /
• pp.86-92
• /
• 2009

A driver-vehicle model means the integrated dynamic model that is able to estimate the steering wheel angle from the driver's desired path based on the dynamic characteristics of the driver and vehicle. Autonomous driving robot for factory automation has individual four-wheels which are driven by electronic motors. In this paper, the dynamic characteristics of several four-wheel steering systems with the simultaneously steerable front and rear wheels are investigated and compared by means of the driver-vehicle model. A diver-vehicle model is proposed by using the PID control to velocity and trajectory of control autonomous driving robot. To determine the optimum speed of a autonomous driving robot, steady-state circle simulation is carried out with the ADAMS program and MATLAB control model.

• 대한전기학회논문지:시스템및제어부문D
• /
• 제51권1호
• /
• pp.32-38
• /
• 2002

This paper describes a development of a portable physical activity monitoring system using two accelerometers to quantify physical activity. The system hardware consists of two piezoresistive accelerometers, amplifiers with gain of 30, lowpass filters with cut-off frequency of 15Hz, offset control circuits, one-chip microcontroller and flash memory card. In order to evaluate the performance of the system we acquired 3 channel data at 32 sample/sec from body-fixed accelerometers in chest and right upper leg. And then the acquired data were processed by MatLab on personal computer. We tried to distinguish not only fundamental actions which are steady-state activities such as standing, sitting, and lying but also dynamic activities with walking, up a stairway, down a stairway, and running. Five subjects participated the evaluation process which compare the video data with the measured data. As a result, the activity classification rate of 90.6% on average was obtained. Overall results showed that the steady-state activities could be classified from the low component of 3-axis acceleration signal and dynamic activities could be distinguished from frequency analysis using wavelet transform and FFT. Finally, we could find that this system can be applied to acquire and analyze the static and dynamic physical activity data.